Today, I had sex with a guy I've liked for ages. We were lying in his bed afterwards, and he mumbled the word "happy". I thought it was really sweet, until he repeated himself. "My girlfriend probably won't be too happy about this." FML

Well, if he is old enough to have a job, it's very easy to hide things like that. I can honestly say that I don't know what each of my colleagues' relationship status is. I had a similar experience. I worked alongside a girl for quite some time without knowing if she had a girlfriend/boyfriend or not. She asked me on a date, so I assumed she didn't. After going on a few dates, we had sex. I commented on a painting afterwards and she replied, "My husband made it last year." I guess she got the lesbian experience she was looking for. I just felt like a complete ass.

There's a huge difference between not knowing or caring about your coworkers relationship status and not knowing the relationship status of someone you /like/. One is minding your own business in a professional environment. The other is just not giving a shit until it's too late.

Quite easily actually. I worked with a guy I had a crush on for 3 months before I found out he had a girlfriend of 2 years, we always had a few drinks after work and just talked absolute shit (those conversations that would put you in a mental hospital if anyone heard) it wasn't untill we had a talk about him leaving for the army that he said "I'm thinking about leaving my girlfriend, I don't think she'd stay faithful while I'm away and she's really against me joining" that I found out ( if anyone cares, yes, he left her and yes, he's mine now)

Oh... I'm sorry. You should be honest and tell his girlfriend if he doesn't. It's not your fault but you should make sure she knows so that she won't get hurt... But either way, it's your choice. Good luck with whatever you do:)

Don't do that. Peope get hurt like that. I mean physically hurt. Cut contact with the guy and go on with your life. People assume that when they tell the significant other the truth they will appreciate it. Sometimes they believe their lovers lie and turn on you. Unless it's a personal friend of yours, just walk away from them both.

#67 Then it's there choice if to leave the man or stay. They might get mad but, at least there aware. If someone told me my boyfriend was cheating on me, I would probably be grateful for them. At least give them a head up. If I ever find out my boyfriends cheating on me and someone who knew didn't tell me, I WOULD blame them then. Especially if they were the one he was cheating with. Then Hell would break loose.

"Guy I've liked" doesn't equal "Guy I'm in a relationship with". If you've known him for 'ages', then you'd know he has a girlfriend, and therefore, he's a cheater, and you have no respect for relationships. I don't care that this will get downvoted, but for me, it's a big YDI, because you just slept with him, because you "like" him, but aren't in a relationship with him? You're fuck buddies?

You'll probably get downvoted, but I agree.
Without getting in the "fuckbuddies" debate (people can do whatever they like afterall), OP clearly saw that as a romantic sutuation, which doesn't really make sense considering she doesn't seem to know the guy so well and isn't even in a relationship with him.